ARTICLE pubs.acs.org/IECR
Calcined Mg Al Layered Double Hydroxides for Uptake of Trace Levels of Bromate from Aqueous Solution Ramesh Chitrakar,* Akinari Sonoda,* Yoji Makita, and Takahiro Hirotsu Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu 761-0395, Japan ABSTRACT: Layered double hydroxides of Mg and Al (Mg Al LDH) were synthesized with OH , Cl , or CO32 as charge compensating anions. The materials were calcined at 500 °C in air. The bromate uptakes of calcined and uncalcined LDH materials were investigated from 0.78 μmol/dm3 bromate solution. Results show that uncalcined LDH exhibited no bromate uptake, although calcined LDH exhibited bromate uptake: the initial bromate concentration of 0.78 μmol/dm3 decreased to less than 0.078 μmol/dm3, the maximum contaminant level established for bromate in drinking water. The rate of bromate uptake of calcined LDH was slow to attain equilibrium in 48 h. The bromate uptake data of calcined LDH agreed well with the Freundlich model.
1. INTRODUCTION Bromate (BrO3 ) is not commonly found in water, but it is formed as a byproduct after ozonation of drinking water containing bromide (Br ).1 Bromate is also formed when chlorinated water is exposed to sunlight2 or when water is treated with concentrated hypochlorite.3 In Japan, bromate concentration of 1.0 μmol/dm3 was reported in an experimental ozonation plant; also, the maximum bromate concentration of 0.23 μmol/dm3 was detected in a drinking water treatment plant.4 In the United States, bromate was also found in both surface waters and groundwater at concentrations of 0.02 1.6 μmol/dm3.5,6 Several reports of scientific studies have described the carcinogenic properties of potassium bromate exposure via drinking water in animals;7 the chemical was shown to be a carcinogen in multiple studies, including kidney tumors and thyroid tumors. The International Agency for Research on Cancer (IARC) classified bromate in group B-2 (as a possible human carcinogen)7 and established a maximum contaminant level of 0.078 μmol/ dm3 (10.0 μg of BrO3/dm3) for bromate in drinking water.1 Once bromate is formed in water, it will not be removed easily by conventional water treatment processes. A wide range of processes using activated carbon,8 10 TiO2,11,12 membranes,13 16 iron nanoparticles,17 20 akaganeite (β-FeOOH),21,22 amorphous aluminum hydroxide,23 chitosan-based molecularly imprinted polymer (MIP) and sol gel adsorbent (Fe2O3 3 Al2O3 3 nH2O),24 catalysts, 25 and layered double hydroxides (LDH),26,27 were investigated for bromate uptake from aqueous solutions. In the case of layered double hydroxides (LDH), the study mainly addressed the uptake of oxo anions including bromate at a concentration of 0.40 mmol/dm3. The bromate removal efficiency was F > Cl > Br > I for monovalent anions. The more strongly bound anions were those with the smaller ionic radii, engendering decreased spacing between the brucite layers.32 It was expected that exchange of BrO3 with CO32 of Mg Al LDH(CO3) did not occur. The ionic radius of BrO3 (0.191 nm) is larger than that of either OH (0.137 nm) or Cl (0.181 nm); it is expected that the exchange of BrO3 with OH or Cl of LDH is very difficult, as reported.32 The Mg Al LDH(A) samples after calcination at 400 or 500 °C exhibited a rapid decrease of bromate concentration from 0.78 μmol/dm3 to less than 0.078 μmol/dm3. The calcined LDH samples at 400 500 °C adsorbed anions from the aqueous solutions because of reconstruction of the LDH structure.30 The calcined Mg Al(A) samples after bromate uptake recovered the original layered structure of LDH, as shown by the X-ray diffraction (XRD) patterns (Figure 1c). The diffraction peaks in Figure 1c were sharper than those of original Mg Al LDH(A) samples (Figure 1a), similar to the results reported for the calcined Mg Al LDH after immersion in water.33 The pH of the solution increased from 6.0 to 9.6 after the calcined Mg Al(A) was stirred with bromate solution for 72 h. An increase of pH value of the solutions after contact with calcined LDH samples might be attributed to the consumption of protons during reconstruction of the LDH structure.34 Effect of Contact Time on Bromate Uptake. The effect of contact time on bromate uptake of calcined Mg Al(A) samples
Figure 3. Effect of contact time on bromate uptake. (4) Calcined Mg Al(OH), (O) calcined Mg Al(Cl), (0) calcined Mg Al(CO3), sample = 0.250 g, volume = 250 cm3, and concentration of BrO3 = 0.78 μmol/dm3.
is shown in Figure 3. The first withdrawal decreased the suspension volume 2% and the last decreased it 20%. The volume of the sampled solution and the mass of the solid can be determined as a check on whether the original solution/solid ratio in the batch system is maintained. It was demonstrated that the solution/solid ratio was not altered to any considerable degree by repeated sampling in a batch system.35 When a magnetic stirrer is used for mixing in a batch system, there will be attrition of solid particles, and it can be expected that small particles are suspended in the aliquots taken for analysis. Rapid filtration was done to separate the solid and liquid phases after suspensions were removed. A 5 10 s sampling time was taken, including connection of the syringe to the filter holder (0.45 μm syringe driven filter unit). The clear filtrate was analyzed. In any process where particles are stirred or mixed in vessels, attrition occurs. The fragments detached from particles undergoing attrition in a stirred vessel were of approximately constant size and independent of the time during which attrition had occurred.36 Lazaridis and Asouhidou37 reported no risk of attrition of the calcined Mg Al LDH particles at a higher agitation speed in a batch experiment of chromium(VI) adsorption. The bromate concentration decreased from the initial 0.78 μmol/dm3 to 0.17 μmol/dm3 (78% removal efficiency) within 4 h, followed by a much slower decrease in bromate concentration to less than 0.078 μmol/dm3 over 48 h reaction time (94% removal efficiency). Of the total bromate uptake observed during 72 h, a 48 h period was chosen to establish uptake equilibrium. This period was used during further experimentation. A similar result was also reported on calcined Mg Al LDH(NO3) for uptake of trace levels of arsenate and selenate. Although the initial uptake occurred rapidly, it took 48 72 h to attain equilibrium.38 The reconstruction process of calcined LDH samples to original LDH structure was slower in aqueous solution. A longer contact time (24 h) was necessary.29 Effect of pH on Bromate Uptake. The bromate uptake of calcined Mg Al(A) from 0.78 μmol/dm3 bromate solution was evaluated at initial pH 4.0 10.5. Data show that the amount of bromate uptake was nearly the same at the pH range 4.0 9.0, with a bromate adsorption capacity of 0.65 0.75 μmol/g. Calcined Mg Al(OH or Cl) samples exhibited slightly higher amounts of bromate uptake than calcined Mg Al(CO3). Yang et al.38 also reported that the initial solution pH did not significantly influence the uptake of trace levels of arsenate and selenate on calcined Mg Al LDH(NO3) at a pH range of 4.0 10.0. 9282
dx.doi.org/10.1021/ie1023468 |Ind. Eng. Chem. Res. 2011, 50, 9280–9285
Industrial & Engineering Chemistry Research
ARTICLE
Figure 4. Freundlich adsorption isotherms of bromate. (4) Calcined Mg Al(OH), (O) calcined Mg Al(Cl), (0) calcined Mg Al(CO3), sample = 0.05 g, volume = 50 cm3, concentration of BrO3 = 0.78 7.8 μmol/dm3, and contact time = 48 h.
Figure 5. Change in bromate concentration with solid-to-solution ratio. (4) Calcined Mg Al(OH), (O) calcined Mg Al(Cl), (0) calcined Mg Al(CO3), sample = 0.02 0.12 g, volume = 100 cm3, initial concentration of BrO3 = 0.78 μmol/dm3, contact time = 48 h, and final pH 9.2.
Table 2. Freundlich Isotherm Constants for Samples sample
Kf
n
R2
calcined Mg Al(OH) calcined Mg Al(Cl)
24 22
1.0 1.0
0.979 0.985
calcined Mg Al(CO3)
17
1.1
0.997
The calcined Mg Al(OH or Cl) samples were more stable than calcined Mg Al(CO3) during bromate uptake. Dissolutions of Mg and Al from the former two samples were less than 0.10 wt % throughout the studied pH range. In the case of calcined Mg Al(CO3) at pH 4.0, the dissolutions of Mg and Al were slightly high (0.70 wt % for each cation), and the amounts decreased to less than 0.10 wt % at pH >5. Bromate Uptake Isotherms. Bromate uptake isotherms were examined on calcined Mg Al(A) samples at initial concentrations of 0.78 7.78 μmol/dm3. Of the two tested adsorption isotherm models of Freundlich and Langmuir, the data showed a better fit to the Freundlich isotherm equation of log qe = (1/n) log Ce + log Kf (Figure 4), where qe signifies the amount of bromate adsorption (μmol/g), Ce denotes the equilibrium bromate concentration in solution (μmol/dm3), Kf represents a measure of adsorption capacity, and n stands for the interaction between the adsorption sites in the sample and bromate ion. The Freundlich parameters are presented in Table 2. The n value of Mg Al LDH(CO3) was 1.1, which is slightly higher than the 1.0 observed for the other two samples. The larger n indicates higher adsorption affinity only, not the capacity. The Kf values with different n values are not strictly comparable.39 The calcined Mg Al(OH) exhibited a higher bromate uptake capacity. The calcined Mg Al(CO3) showing a lower bromate uptake capacity might arise because the calcined sample might contain fewer carbonate anions even though calcination was carried out at 500 °C.38 The Freundlich model was also used to describe the uptake of trace levels of arsenate and selenate on calcined Mg Al LDH(NO3).38 Effect of Solid-to-Solution Ratio on Bromate Uptake. The ability of calcined Mg Al(A) to decrease bromate concentration to a contaminant level below 0.078 μmol/dm3 was examined by varying the masses of the samples. Results of the bromate concentration against the amount of calcined samples are depicted in Figure 5. The samples showed a rapid decrease in bromate concentration at a low solid-to-solution ratio of 0.20 g/dm3. With a further increase in the solid-to-solution ratio to 0.6 g/dm3,
Figure 6. Effect of anions on bromate uptake on calcined Mg Al(Cl): (b) Cl , (2) NO3 , (O) SO42 , (0) CO32 , sample = 0.05 g, volume = 50 cm3, concentration of BrO3 = 0.78 μmol/dm3, and contact time = 48 h.
the calcined Mg Al(OH or Cl) samples showed a further decrease in bromate concentration to lower than 0.078 μmol/dm3. Figure 5 shows that the bromate concentration decreased with increasing mass of the sample up to a certain value, after which the bromate concentration remained nearly constant with further addition of the sample. According to the surface site heterogeneity model,40 the surface is composed of sites with a spectrum of binding energies. At a low adsorbent dose, the sites of all types are entirely exposed for adsorption. Therefore, the surface becomes saturated more quickly. However, at higher particle concentration, the availability of the higher energy sites decreases and a large fraction of lower energy sites become occupied. Therefore, the active adsorption sites are more at a fixed adsorbate concentration. Consequently, a solid-to-solution ratio of 1.0 was sufficient for the quantitative removal of bromate. The calcined Mg Al(CO3) exhibited a slightly lower bromate uptake. With a much smaller solid-to-solution ratio of 0.10 g/dm3, the chitosan-based MIP adsorbent and sol gel adsorbent (Fe2O3 3 Al2O3 3 nH2O) were used to decrease the bromate from the initial concentration of 0.23 μmol/dm3 to less than the accepted level of 0.078 μmol/dm3 in the presence of the nitrate solution.24 Effect of Coexisting Anions on Bromate Uptake. The bromate uptake of calcined Mg Al(Cl) was studied in the presence of coexisting anions such as Cl , NO3 , SO42 , or 9283
dx.doi.org/10.1021/ie1023468 |Ind. Eng. Chem. Res. 2011, 50, 9280–9285
Industrial & Engineering Chemistry Research
ARTICLE
Table 3. Comparison of Bromate Adsorption on Different Materials sample biogenic sulfide
initial bromate (μmol/dm3)
equilibrium bromate (μmol/dm3)
300
mechanism
ref
ca. 0
reduction
5
activated carbon
0.39
0.078
reduction
8
TiO2
2.0
0.46
reduction
11
bioreactor
1.56
